MR imaging can provide high resolution 3D maps of structural and functional information in vivo yet its use of mapping in vivo gene expression is largely unexplored. The major obstacle to its implementation is the low threshold of (super) paramagnetic label detection. We have developed a strategy that relies on synergistic amplification of biological processes to directly map transgene expression. The basic concept and general method relies on 1) uninhibitable internalizing receptors (e.g. engineered transferrin receptor, ETR) and probing the receptor with affinity ligands (e.g. Tf) containing thousands of iron atoms which increase R2/R2 relaxivity upon cellular internalization. In preliminary feasibility studies we have created cell lines stably expressing ETR and are insensitive to iron-induced down regulation. Using ETR targeted holo-Tf containing iron oxide (Tf-MION), we have furthermore shown that 1) cellular uptake of this probe is specific and inhibitable, 2) cellular accumulation of iron from Tf-MION is more efficient for MR imaging when compared to iron from Tf, 3) that the probe accumulates at significantly higher amounts in ETR+ tumors compared to ETR- tumors and 4) that transgene expression can be visualized in vivo by MR imaging (1.5 T) in live animals and by high resolution MR microscopy (7.1T). With our expertise in constructing viral expression vectors we propose to expand this research to test whether the recombinant ETR system can be used as a universal marker gene for MR imaging of gene expression. The hypotheses underlying these experiments is that ETR gene expression will correlate with expression of therapeutic genes when driven by the same promoter and/or being part of a polycistronic vector. Using novel HSV/EBV amplicons we will test this model system in human tumors xenografted into immunocompromised mice. The long-term goal of this research is to extend the capabilities of MR and apply it to in vivo imaging of gene expression.